The present invention relates generally to transparent, energy-absorbing polyurethanes and more particularly to such polyurethanes for use in safety glass laminates with high impact resistance over a wide temperature range.
Safety glass is a well-known term for a glass-plastic laminate designed to reduce the severity of lacerative injuries resulting from glass breakage upon impact. A polymeric film is laminated to a glass sheet so that upon impact sufficient to break the glass, the film adheres to the glass fragments, minimizing their dispersion. To be useful as safety glass, a laminate must have (1) high energy absorption to minimize concussive injuries on impact, (2) high shear and tear strength to prevent rupture of the film by glass fragments, (3) sufficient adhesion between the layers to minimize the dispersion of glass fragments to reduce the potential for lacerative injuries and (4) high optical quality.
Commercial safety glass, particularly for automobile windshields, is typically a trilayer laminate comprising two sheets of glass with an intermediate layer of plasticized polyvinyl butyral. However, there is a trend toward substituting other polymeric interlayer materials for polyvinyl butyral.
In U.S. Pat. No. 3,509,015, Wismer et al, describe a safety glass laminate with a cast-and-cured-in-place polyurethane interlayer prepared by the reaction of an organic diisocyanate and a curing agent with a prepolymer which is formed by the reaction of an organic diisocyanate and a poly(oxypolymethylene)glycol. Suitable curing agents include polyols, especially those with at least three hydroxyl groups, and preferably in conjunction with a diol, and polyamines, preferably utilized with a polyol. The reaction mixture is heated, degassed, placed in a casting cell, cured and the resulting laminate is tested. Impact resistance of the laminate, measured as the height from which a 1/2 pound steel ball may be dropped with the laminate withstanding the impact, is acceptable.
In U.S. Pat. No. 3,620,905, Ahramjian discloses colorless, optically clear, thermoplastic polyurethanes suitable for use in safety laminates prepared from diisocyanate-dicyclohexylmethane, a polyether or polyester glycol, and a diol having a molecular weight less than 250. The polyurethanes may be prepared by one-shot, quasi-prepolymer, or conventional prepolymer procedures, all of which are well-known in the art.
U.S. Pat. No. 3,764,457 to Chang et al discloses safety glass laminates comprising a thermoplastic polycarbonate urethane formed from a cycloaliphatic diisocyanate, a monomeric aliphatic diol, and an aliphatic polycarbonate such as polyoxyethylenecarbonate glycol.
In U.S. Pat. No. 3,900,446, McClung et al disclose laminated glazing units containing polyurethane interlayers prepared from an isomeric mixture of 4,4'-methylene-bis-(cyclohexylisocyanate), a polyester having a melting point above 42.degree. C. which is the condensation product of a dicarboxylic acid and a dihydric compound, and an alpha-omega diol having from 2 to 10 carbon atoms. The two-step method for making the polyurethanes involves prepolymer preparation and polymer preparation.
U.S. Pat. No. 3,900,655 to Wolgemuth et al describes laminated safety glass made with a thermoplastic interlayer which is the polyurethane reaction product of a cyclic nitrile carbonate and at least one hydroxyl-containing compound such as a polyalkylene ether or polyester glycol or a diol having primary or secondary hydroxyl groups and a molecular weight less than 250. Such polyurethane elastomers can be prepared by a variety of methods well-known in the art such as one-shot, quasi-prepolymer or full prepolymer procedures.
In U.S. Pat. No. 3,931,113, Seeger et al disclose polyester urethanes having superior properties for use in safety glass windshields which are formed from a cycloaliphatic diisocyanate, a low molecular weight diol, and a hydroxy terminated polyester of polycaprolactone, poly(butylene adipate), poly(butylene azelate) or mixtures thereof. These urethanes are preferably prepared by the one-step bulk polymerization method which provides a flexible polymer having a random distribution of components.
In U.S. Pat. No. 4,024,113, Ammons discloses energy-absorbing safety glass laminates comprising a polycarbonate urethane formed from a cycloaliphatic diisocyanate, a low molecular weight diol, and a special polycarbonate diol synthesized from a mixture of linear aliphatic and cycloaliphatic diols. The polycarbonate urethanes can be prepared either by the "one-shot" bulk polymerization method or by the prepolymer method.
U.S. Pat. No. 4,035,548 to Chang et al discloses safety glass laminates containing energy-absorbing interlayers made from a poly(lactoneurethane) in which the molecular weight and structure of the lactone moiety are carefully controlled in order to obtain optimum energy-absorbing and optical properties. The "one-shot" method of polymerization is preferred to the prepolymer method because of its simplicity and the lower initial viscosity of the reactants.
Many of the reference relating to transparent, energy-absorbing polyurethanes for interlayers in safety glass laminates teach that cycloaliphatic diisocyanates are preferred, particularly 4,4'-methylene-bis-(cyclohexylisocyanate), because of their contribution to colorlessness, transparency and impact resistance. Unfortunately, these diisocyanates are rather expensive. Mixtures with minor amounts of less expensive diisocyanates, such as toluene diisocyanates (TDI) or diphenyl methane diisocyanate (MDI) and other aromatic diisocyanates, are usable only if the amount of aromatic diisocyanate employed is carefully controlled to avoid yellowing, translucence and reduced impact resistance.